There is of course more than one reason for its popularity.
But the dominant one is undoubtedly the
idiosyncrasy of the author, which is revealed to
an extent unusual in a 'dictionary'.
-Sir Ernest Gowers,
Preface
to the Revised Edition

Here in the States we have our beloved Strunk
& White to give us guidance on matters grammatical, and it remains
an indispensable reference work, even in its original form. The British
counterpart to Elements of Style is this unique work by H.W. Fowler,
minimally revised and edited by Sir Ernest Gowers. It too remains
useful, though many entries have grown dated, but it is so idiosyncratic
and amusing that even the most obsolete of Fowler's rulings and admonitions
are worth reading if for nothing more than simple amusement. Here
are just a few of the more enjoyable ones that I found :

continental. 'Your mother,' said Mr.
Brownlow to Mr. Monks in Oliver Twist, 'wholly given up to
continental frivolities, had utterly forgotten
the young husband ten years her junior.' This use of
continental reflects the common belief in
England that the Continent, especially France, offers
unwonted opportunities for gaiety and self-indulgence.
It persists in such expressions as c. Sunday,
c. cabaret, now not necessarily in the pejorative
sense intended by Mr. Brownlow but suggesting
either envy or reprobation, or a mixture of both,
according to the taste of the user. Such feelings
toward what we suppose to be the continental
way of life have no doubt changed with the
mellowing of Victorian prudery, but are unlikely
to disappear so long as we are not allowed to
gamble where we please or to drink whenever we are
so disposed.

paragraph. The purpose of paragraphing
is to give the reader a rest. The writer is saying to him:
'Have you got that? If so, I'll go on to the
next point.' there can be no general rule about the most
suitable length for a paragraph; a succession of
very short ones is as irritating as very long ones are
wearisome.

reactionary. 'Except for its technical scientific
sense, to which it would be a mercy if it were
confined, reactionary is a word so emotionally
charged as to be little more than a term of abuse'
(Evans). That is no less true of Britain than
of America. The word derives its pejorative sense
from the conviction, once firmly held but now badly
shaken, that all progress is necessarily good.

split infinitive. The English-speaking
world may be divided into (1) those who neither know nor
care what a split infinitive is; (2) those who do
not know, but care very much; (3) those who know
and condemn; (4) those who know and approve; (5)
those who know and distinguish.

1. Those who neither know nor care are the vast majority,
and are a happy folk, to be envied by
most of the minority classes.

As Sir Ernest says in the epigraph to this review, that's not a style
we're used to finding in dictionaries.

Meanwhile, unbeknownst to the rest of us, there's apparently a
battle amongst lexicographers, of some years duration, over whether
such usage manuals should be prescriptive or descriptive. If
descriptive, they would merely describe what the masses have adopted as
common usage. If prescriptive, the author
tries to offer guidance and to influence future usage. Fowler
seems, to at least this non-professional, to have struck a nice balance
between the two. He certainly has pet peeves (more than a few) and
quite forcefully argues for spellings and definitions which he feels ought
to be either stuck to or adopted, but he is also sufficiently democratic
to recognize that many of these struggles, though he might have favored
a different result, had already been decided to his disfavor.
Here is but one example :

contact. The use of c. as a verb (get
into touch with) gave no little offence when it first appeared
here from America. But convenience has prevailed
over prejudice, and the dictionaries now give it
full recognition : after all, it is an ancient and
valuable right of the English people to turn their
nouns into verbs when they are so minded.

Given this realistic attitude, one assumes he would have been able to
gracefully handle the fact that many of his suggestions have gone unheeded.

At any rate, from what the reviewers
have to say about the most recent version of the Modern English
Usage, Fowler's successor, Robert
Burchfield, would appear to have produced a work that is not only overly
descriptive, but that tends
to vacillate over certain usages, as if Burchfield is unwilling to
have events prove his judgments wrong in the future. There is no
such waffling in the original, and it is a much better book for the firmness
of its author's often hilarious opinions. This is one of those books
that belongs on every desk in the English-speaking world, alongside Strunk
& White and the OED, you'll refer to it often, but browse
for pleasure even more frequently.